Re: [Haskell-cafe] Filtering on data constructors with TH

On 6/1/06, Christophe Poucet wrote:

That's why today I have created an automated derivation for data constructor filtering. As I started coding someone mentioned that something similar can be done with list comprehensions, so I'm not certain about the scope of usefulness, however personally I have found the need for this at times.

data T = A Int | B String deriving Show test1 = [A 3, B "hello", A 5] test2 = [x | x@(A _) <- test1] The key here is that pattern match failure in a monad calls fail: http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Afail and fail in the List monad is [].